The invention pertains to improvements to equipment for testing microcircuits. The manufacturing processes for microcircuits cannot guarantee that every microcircuit is fully functional. Dimensions of individual microcircuits are microscopic and process steps very complex, so small or subtle failures in a manufacturing process can often result in defective devices. For this reason, microcircuit testing has become widely practiced, in both the semiconductor manufacturing and the circuit board assembly industries.
While the cost of a microcircuit is frequently quite small, mounting one on a circuit board adds substantial value due to the cost of the circuit board and the manufacturing cost itself. Installation usually involves soldering the microcircuit onto the circuit board, and is not usually reversible. Once mounted on a circuit board, removing a microcircuit often ruins the circuit board. Thus, if the microcircuit is defective, the circuit board itself is probably ruined as well, meaning that the entire value of the circuit board at that point is lost.
For all these reasons, a microcircuit is usually tested before installation on a circuit board. Each microcircuit must be tested in a way that identifies nearly all defective devices, but only rarely identifies good devices as defective. Either kind of error adds cost to the overall manufacturing process.
Microcircuit test equipment itself is quite complex. First of all, the test equipment must make accurate, low resistance, temporary duty, non-destructive, contact with every one of the small, closely spaced microcircuit terminals. Because of the small size of microcircuit terminals and the small spacing between adjacent pairs, even small alignment errors between the test equipment contacts and microcircuit terminal pads will result in incorrect connections. Connections to the microcircuit that are misaligned or otherwise incorrect will cause the test equipment to identify the microcircuit, that is, the device under test (DUT), as defective, even though the reason for the failure is the defective connections rather than defects in the DUT itself.
A further problem in microcircuit test equipment arises in automated testing, where a single system may test 100 or more DUTs a minute. The sheer number of tests can cause wear on the test equipment contacts as the DUT terminals mate with the test equipment contacts and wipe against them. This wiping often removes particles from the DUT terminals. The particles can then build up on the test equipment contacts. The DUT packages may also transport contaminants such as oils or mold release materials derived from DUT fabrication processes, to the test equipment contacts and other components of the test equipment.
The particles and other contaminants may eventually build to a point where they interfere with the electrical connections between the DUT terminals and the test equipment contacts during testing, which may result in false indications that the DUT is defective. The particles may also create leakage paths between adjacent test equipment contacts, also leading to false indications that the DUT is defective.
One particular type of microcircuit often tested before installation has a relatively large, centrally located ground (CG) terminal on a flat, bottom surface of the microcircuit package. The microcircuit signal and power (S&P) terminals surround the CG terminal in a predetermined array. Microcircuit packages having this configuration of terminals may be called CG packages.
FIGS. 1A and 1B show the current state of the art for a housing 15 and related structure that collectively form a part of a test system for DUTs configured as CG packages. In a complete test system, housing 15 fits between an alignment plate and a load board (neither shown in FIGS. 1A and 1B). Housing 15 is made from an insulating material. This configuration of alignment plate, load board, and housing 15 is well known in microcircuit testing technology.
A complete test system includes an alignment plate having an aperture whose walls precisely position the DUT in the X and Y axes directions relative to test contacts (not shown) that are held in an array of slots 21. The alignment plate aperture forms the walls of a test well or cavity in which the DUT is placed by a loader or handler during a test procedure. Typically, the array of slots 21 comprises four quadrants, one of each of which is positioned adjacent to one of the four walls or edges of the alignment plate aperture along the Z axis.
Housing 15 has in a surface 29, an aperture 28 for receiving a CG terminal insert 25 shown in FIG. 1B. Insert 25 has a CG test contact 13 that projects from and is centrally located in the top surface 16 of insert 25. Test contact 13 makes electrical connection with the body of insert 25. Test contact 13 is mounted in insert 25 with resilience along the Z axis relative to insert 25. Retention projections extend from the walls of aperture 28 and resiliently deflect slightly during installation of insert 25. After installation, these projections project into recesses 31 of insert 25 to retain insert 25 within aperture 28. The projections do not restrict insert 25 from shifting slightly along the Z axis after installation.
The surface 29 of housing 15 and an upper surface 16 of insert 25 form the floor or bottom on which the DUT rests. Surface 16 projects slightly above the plane of surface 29. Surface 16 has a footprint that puts it within all of the S&P terminals on the DUT surrounding the CG terminal on the DUT surface with the CG terminal. This provides clearance for access to the S&P terminals.
The top end of test contact 13 when not deflected in the Z axis direction, projects slightly above surface 16 of insert 25 as seen in FIG. 1b. The bottom end 27 of insert 25 mechanically rests on and is supported in the Z axis direction by a surface of the load board having an electrical contact for insert 25. In this way, the CG terminal of the DUT is electrically connected to the load board. In this configuration of housing 15 and insert 25, insert 25 is pressed against the load board only when the loader is pressing a DUT against test contact 13 and surface 16.
Slots 21 of housing 15 hold S&P test contacts that project from slots 21 toward surface 29. The center to center spacing of each slot 21 aligns each S&P test contact with its associated S&P terminal on a DUT to allow the S&P test contact to make good electrical contact with its associated S&P terminal for testing. As with CG test contact 13, each S&P test contact is designed to resiliently deflect in the Z axis direction.
To begin a test, a loader moves the DUT into a prescribed position for testing (test position). In the test position, the DUT's CG terminal presses against test contact 13, the S&P terminals of the DUT press against the S&P test contacts, and the surface of the DUT's CG terminal surrounding contact 13 presses on surface 16. In the test position, all of the S&P test contacts are slightly deflected in the Z axis direction by force the respective DUT terminals apply. This force on the individual test contacts from their respective DUT terminals provides assurance that all of the S&P contacts make good electrical contact with the DUT terminals. This arrangement makes consistent contact force between each DUT S&P terminal and the associated test contact likely, and prevents damage to either.
In the test position, the load board holds insert 25 in a Z axis position that provides a hard stop for the DUT Z axis position and prevents overtravel of the DUT. This arrangement provides assurance that the Z axis position of each DUT is nearly identical as each is tested. The force of the CG terminal against test contact 13 is controlled by the resiliency of the mounting of contact 13 in insert 25 and is designed to provide proper contact pressure (force per unit area) and wiping between insert 13 and the DUT CG terminal during testing.
In current test equipment designs, the mounting of insert 25 does not strongly force insert 25 into contact with the load board when a DUT is not in the test position. As the DUT moves into the test position and is pressed against insert 25, the loader's pressure on the DUT often shifts insert 25 along each of the X, Y, and Z axes a very small amount within aperture 28 and on the load board.
This shifting when repeated for many thousands of DUT insertions, can create significant wear on the insert 25, the housing 15, and the load board pad in contact with insert 25. The effect of this wear is to require replacement of these components, during which time the test equipment cannot be used. The wear also creates debris that as explained above, may interfere with the testing process.